There are other styles like sun style, wu style, etc are practised. Which style is better for which class of people? i.e. which style is better for elders while which style is better for stress relief?

Beginner,Taijiquan is one art. There is no one style of it that is "better" than another.The best that can be said is that there are styles that some people like better than they do the others.Each named style, and even further than that each Master, has his own way of doing things. It's better to find a "style" that you enjoy more than looking for one that is "better" than the others. It's a matter of personal taste which one will work best for you, not the style itself. The best advice I can give you is to visit the schools that you have interest in and meet the instructor that you would train with. Get to know them personally and if you like them and if they are qualified (a whole 'nother discussion) then work with them for a while and see if you enjoy what they do and how they teach it.What matters the most in the long run is to enjoy what you're doing and who you're doing it with.All paths will lead you to Taijiquan, some will be better for you, personally, than others.

I would suggest CMC 37 Step Taichi (Cheng Man Ching) I found it is suitable for or ''better for the elders'' folks. I have learned this style and found that the form does not require too much physical demand.

For ''better in stress'' reduction, I still recommend CMC 37 or Yang 47 performed at a slower pace.

But you need to check out the teacher as some are teaching it, the 2 style mentioned above, as a form of Martial Art or self-defenced.

I would have to say that the Yang system if taught completely should cover all that you want in the system be it martial or simple exercise.

Which ever style you choose, I would always advocate learning a long form over a short form. The long forms are complete for the health benefits, etc wheras the short forms were created to make the systems easier to learn.